Currently, second and third generation systems are mainly utilized. In recent years, standardization of Long Term Evolution (LTE) regarded as a fourth generation system is in progress.
As the LTE system basically supports packet services (PS) only, the LTE standard has proposed Circuit-Switched Fallback (CSFB) to provide voice call services.
FIG. 1 illustrates a basic system architecture supporting CSFB (or SMS over SGs).
As shown in FIG. 1, CSFB refers to a service in which, when a voice call is made to a user equipment (UE) camping on the LTE system, the UE is handed over to a legacy 2G/3G system for voice call connection. To achieve this, a new interface SGs is introduced between Mobility Management Entity (MME) 120 serving as a control node on the LTE network (Evolved Packet Core, EPC) and Mobile Switching Center (MSC) 110 serving as a control node on the CS network (refer to 3GPP TS 23.272). In addition to the voice call service, as the SMS service is mainly used in the CS network, the SMS service may be realized using the SGs interface, which is referred to as SMS over SGs.
Meanwhile, to cope with significantly increased network traffic due to popularization of smartphones and introduction of new services, congestion control technology has been introduced to the mobile communication network. Specifically, in the LTE system (EPC), upon network congestion, the MME 120 may apply a back-off timer so as to prohibit the UE from performing mobility management (MM) or session management (SM) signaling for a given duration. That is, when the UE receives a back-off timer from the MME 120 (or base station or Evolved Node B (eNB)), the UE cannot send a Non Access Stratum (NAS) request (e.g. attach request, TAU request, or service request), except for special occasions, until expiration of the back-off timer (refer to 3GPP TS 23.401).
When the network operator simultaneously operates an LTE system and a 2G/3G system, a UE may selectively connect to the LTE system or the 2G/3G system according to circumstances. To control this according to network situations, a preset core network node (MME, SGSN or MSC 110) may set a Service Profile Identifier (SPID) for Radio Access Technology/Frequency Selection Priority (RFSP) and send the SPID to a preset RAN node (eNB, RNC or BSC), and the RAN node may determine priorities for Radio Access Technologies (RAT) to be selected in idle mode on the basis of the RFSP index and notify the UE of the determined priorities.